1. Field of the Invention
The present invention relates to a lens barrel having an image-shake correction function, which is used in an image pickup apparatus or the like, and more particularly to a zoom lens barrel having an image-shake correction function, which is used in an image pickup apparatus such as a video camera or a digital still camera which performs recording of a moving image and a still image by using a solid-state image pickup element such as a CCD.
2. Description of Related Art
Image pickup apparatus of the type in which a solid-state image pickup element such as a CCD is disposed in an image forming plane have been widely used as video cameras, digital still cameras or the like. In general, the larger the number of pixels per image pickup element such as a CCD used in such image pickup apparatus, the larger the size of the image pickup element and the higher the cost of the same. In terms of these considerations, numerous image pickup apparatus for general domestic use employ a so-called 1/4-inch size CCD whose diagonal length is approximately 4 mm or a so-called 1/3-inch size CCD whose diagonal length is approximately 6 mm. These CCDs generally have 300,000 to 400,000 pixels.
Since the image size of such image pickup apparatus is small compared to the diagonal length, 43 mm, of the image size of a silver-halide camera using a so-called 135 film, it is possible to greatly reduce the size of a lens for the image pickup apparatus compared to the lens of the silver-halide camera if both lenses have the same angle of view. In practice, video cameras using 1/4-inch size CCDs are in general provided with zoom lenses having a zoom ratio of 10 and an overall length of approximately 50 mm.
However, if such a zoom lens is mounted to a video camera or a digital still camera having a small size and a light weight, there is the problem that it is difficult for a photographer to obtain a stable image owing to a vibration of a hand applied to the image pickup apparatus, particularly if the photographer performs photography with the focal length of the image pickup apparatus being set to a comparatively long focal length side. To obtain a stable image, various image-shake preventing systems have heretofore been proposed. This kind of image-shake preventing system is capable of not only eliminating a harmful image shake due to a vibration of a hand applied to the image pickup apparatus, but also producing a great image-shake correction effect even in a situation in which no harmful image shake can be easily eliminated even with a tripod, for example, when photography is being performed on a travelling vessel or vehicle.
This kind of image-shake preventing system at least includes vibration detecting means for detecting a vibration and image-shake correction means for performing a predetermined correction to prevent occurrence of an image shake, according to information about the detected vibration. An angular-acceleration detector, an angular-velocity detector, an angular-displacement detector and the like are known as the vibration detecting means.
Optical correction means and electronic correction means are known as the image-shake correction means. The optical correction means is arranged to bend a photographing optical axis by using a variable angle prism or by shifting part of a photographing optical system in a plane perpendicular to the photographing optical axis (in a direction perpendicular to the photographing optical axis), while the electronic correction means is arranged to sequentially shift, each time an image shake occurs, a cutout position at which to cut out a particular area to be actually used as a picture toward a position at which the image shake can be corrected, in a video camera arranged to cut out such particular area from obtained picked-up image information. However, the latter image-shake correction means can provide a method of correcting continuous images in a moving image, but does not at all function as still-image correction means.
In general, the optical correction means is capable of effecting an image-shake correction for a vibration of not greater than an angle determined as an image-shake correction angle of a camera, irrespective of the focal length of a lens. Accordingly, the optical correction means is capable of realizing the performance of eliminating an image shake to a practically sufficient extent even if the zoom lens of the camera is set to a long focal length, i.e., a telephoto side (a long focal length side).
FIGS. 9(A), 9(B) and 9(C) are views illustrating the relation between the focal length and the vibration angle of a camera in the form of the position of a subject in a picture. Referring to FIG. 9(A), if the camera is placed at the position indicated by 112, the optical axis of the lens of the camera extends along a line 113 and an image of the face of a person 111 who is a subject is picked up approximately in the center of the picture. If the camera is rotated from this state through an angle of "a" degrees owing to a vibration, the position of the camera becomes as shown by 114 and the position of the optical axis becomes as shown by 115.
FIGS. 9(B) and 9(C) respectively show the positions of the subject in the picture when the camera is at the positions 112 and 114. FIG. 9(B) shows the state of the picture obtained when the zoom lens is set to its wide-angle end (the end of its short focal length side), and FIG. 9(C) shows the state of the picture obtained when the zoom lens is set to its telephoto end (the end of its long focal length side). In FIGS. 9(B) and 9(C), reference numeral 116 denotes the subject viewed in the picture, reference numerals 117 and 119 denote the pictures obtained when the camera is at the position 112, and reference numerals 118 and 120 denote the pictures obtained when the camera is at the position 114.
As can be seen from FIGS. 9(A), 9(B) and 9(C), even if the vibration angle of the camera is the same "a" degrees, it is natural that as the focal length of the zoom lens becomes longer, a more harmful image shake occurs in the picture. Accordingly, the optical correction means is more remarkably effective when it is combined with a zoom lens having a longer focal length on its telephoto side.
FIGS. 10(A), 10(B), 10(C), 11(A), 11(B), 12 and 13 show an arrangement using a variable angle prism, as one example of image-shake correction means. FIGS. 10(A)-10(C) show the arrangement of the variable angle prism itself. In FIG. 10(A), reference numerals 121 and 123 denote glass plates, and reference numeral 127 denotes a bellows part made of a material such as polyethylene. A transparent liquid 122 such as silicone oil is enclosed in the portion surrounded by the glass plates 121 and 123 and the bellows part 127.
Referring to FIG. 10(B), the two glass plates 121 and 123 are disposed in parallel with each other, and the angle at which a ray 125 enters the variable angle prism is equal to the angle at which the ray 125 exits from the variable angle prism. In contrast, if the two glass plates 121 and 123 make a particular angle such as that shown in FIG. 10(A) or 10(C), the passing ray is bent with a particular angle, like either of the rays 124 and 126 shown in FIGS. 10(A) and 10(C). Accordingly, it is possible to eliminate an image shake by controlling the apex angle of the variable angle prism which is disposed in front of a lens, so that the passing ray can be bent by an amount equivalent to an angle at which a camera is tilted by a cause such as a vibration of a hand which holds the camera.
FIGS. 11(A) and 11(B) schematically show the effect of the above-described control for ease of understanding. FIG. 11(A) shows the state in which the variable angle prism is placed in its parallel state and the ray propagates straightforwardly toward the head of the subject. If the ray is bent by driving the variable angle prism by an amount equivalent to a vibration of an angle of "a" degrees, as shown in FIG. 11(B), the photographing optical axis can be kept coincident with the head of the subject.
FIG. 12 is a schematic view showing an actual arrangement example of a variable angle prism unit including the aforesaid variable angle prism, an actuator part for driving the variable angle prism, and an apex-angle sensor for detecting the angular state of the variable angle prism.
Since actual vibrations occur in all directions, the variable angle prism is arranged in such a manner that its front and rear glass surfaces are rotatable about their rotating axes which differ 90 degrees from each other. In FIG. 27, reference numerals having affixed characters "a" denote constituent elements which are provided for producing rotations about one of the two rotating axes, while reference numerals having affixed characters "b" denote constituent elements which are provided for producing rotations about the other rotating axis. The constituent elements indicated by identical reference numerals (excluding the affixed characters "a" and "b") have completely the same function. For this reason, the following description is made using the reference numerals with the respective affixed characters "a" and "b" being omitted therefrom. The constituent elements arranged on the "b" side are partially not shown.
As shown in FIG. 12, a variable angle prism 141 includes the glass plates 121 and 123, the bellows part 127, a liquid enclosed in the bellows part 127, and other associated elements. The glass plates 121 and 123 are integrally attached to corresponding holding frames 128, as by an adhesive. The respective holding frames 128 constitute rotating axes 133 in combination with corresponding fixed components which are not shown, and are rotatable about their rotating axes 133. The directions of the rotating axes 133a and 133b differ 90 degrees from each other. Coils 135 are integrally provided on the respective holding frames 128, and magnets 136 and yokes 137 and 138 are provided on a fixed portion which is not shown.
In such an arrangement, if current is made to flow through either of the coils 135, the variable angle prism 141 rotates about the corresponding one of the rotating axes 133. A slit 129 is provided at the extending end of an arm portion 130 which integrally extends from the holding frame 128, and an apex-angle sensor for detecting the angular state of the variable angle prism 141 is formed by the slit 129 as well as a light emitting element 131 such as an iRED and a light receiving element 142 such as a PSD, both of which are provided on the fixed portion.
FIG. 13 is a block diagram showing an arrangement in which a lens is combined with an image-shake correction apparatus provided with the variable angle prism 141.
The arrangement shown in FIG. 13 includes the variable angle prism 141, apex-angle sensors 143 and 144, amplification circuits 153 and 154 for amplifying the outputs of the respective apex-angle sensors 143 and 144, a microcomputer 145, vibration detecting means 146 and 147 each of which is formed by an angular-acceleration sensor or the like, actuators 148 and 149 each of which is made up of the coil 135, the yoke 138 and the like, and a lens 152.
The microcomputer 145 determines currents to be supplied to the respective actuators 148 and 149, to control the variable angle prism 141 to place it into an angular state optimum for elimination of an image shake in a picture, according to the angular states of the variable angle prism 141 detected by the respective apex-angle sensors 143 and 144 and detection results provided by the respective vibration detecting means 146 and 147. Incidentally, the major elements shown in FIG. 13 are each prepared as a pair of blocks so that control operations for two directions which differ 90 degrees differ from each other can be independently performed.
The arrangement described above is merely one example, and any of the aforementioned kinds of sensors may be used as the vibration detecting means. Although an optical type of sensor using a light emitting element and a light receiving element has been referred to above as a sensor for detecting the apex angle of the variable angle prism, it is also possible to adopt a method of measuring a positional relation between a magnet and a coil by using a magnetic sensor such as a Hall element.
The amplification circuits for amplifying the outputs of the respective apex-angle sensors, which have been described above with reference to FIG. 13, may also be omitted according to the type of sensor. In addition, although the above description has referred to a method of correcting an image shake in an arbitrary direction by rotating two glass plates, which constitute a variable angle prism, about rotating axes which differ 90 degrees from each other, an arrangement for driving one glass plate in an arbitrary direction is also proposed in Japanese Laid-Open Patent Application No. Hei 8-43769 and others.
In such a zoom lens barrel having the image-shake correction means using such a variable angle prism, it is necessary to dispose a large unit in front of a zoom lens (on a subject side) as the variable angle prism, because the diameter of the variable angle prism needs to be determined so that an effective ray on a wide-angle side of the zoom lens is not shaded. This requirement is a hindrance to a further reduction in the entire size of an image pickup apparatus. To solve this problem, the art of disposing a variable angle prism element in the inside of a lens is also proposed. However, in this art as well, since the variable angle prism element which does not directly participate in an image forming operation must be disposed in a photographing lens, the entire optical length of the entire lens can only be reduced to a limited extent, so that it is impossible to avoid an increase in the entire size of an image pickup apparatus.
Similarly, in the field of a zoom lens barrel having no image-shake correction means, a reduction in size is an important problem. FIGS. 14(A) and 14(B) show one example of the aforesaid type of zoom lens barrel, and FIG. 14(A) is a longitudinal sectional view of the zoom lens barrel, while FIG. 14(B) is a longitudinal sectional view taken along line A--A of FIG. 14(A). In the zoom lens barrel shown in FIGS. 14(A) and 14(B), four lens groups 201a to 201d constitute a photographing zoom lens, and the lens group 201a is provided as a fixed front lens, the lens group 201b is provided as a variator lens group which moves along an optical axis 205 to effect a magnification varying operation, the lens group 201c is provided as a fixed afocal lens, and the lens group 201d is provided as a focusing lens group which moves along the optical axis 205 to maintain the position of a focal plane and effect a focusing operation during a magnification varying operation. Reference numerals 203, 204a and 204b denote guide bars which are disposed in parallel with the optical axis 205 to guide the movable lens groups while preventing rotation thereof during their movements.
A DC motor 206 serves as a drive source for moving the variator lens group 201b. The DC motor 206 may also be replaced with a stepping motor or the like. The variator lens group 201b is held by a holding frame 211. The holding frame 211 has a pressure spring 209 and a ball 210 which is pressed in engagement with a screw groove 208a formed around a screw rod 208, by the force of the pressure spring 209. In this arrangement, if the screw rod 208 is rotationally driven by the DC motor 206 through an output shaft 206a and a gear train 207, the holding frame 211 is moved along the guide bar 203 in the direction of the optical axis 205.
In FIG. 14(B), reference numeral 212 denotes a stepping motor. The focusing lens group 201d is held by a holding frame 214. A screw member 213 is integrally secured to a sleeve portion 214a of the holding frame 214, and is screwed onto an externally threaded portion of an output shaft 212a of the stepping motor 212. In this arrangement, the holding frame 214 can be moved along the guide bars 204a and 204b in the direction of the optical axis 205 by the rotation of the stepping motor 212.
In FIG. 14(B), reference numeral 218 denotes an IG meter which drives an iris unit, and reference numeral 220 denotes a camera body to which the zoom lens barrel is secured.
As described above, even the zoom lens barrel having no image-shake correction means needs three actuators, i.e., an IG meter for driving an iris, a zooming motor for driving a variator lens group, and a focusing motor for driving a focusing lens. To reduce the size of the zoom lens barrel, it is important to consider how efficiently and compactly these actuators are to be laid out.
As is apparent from the above description, the zoom lens using the aforesaid variable angle prism has the problem that the size and the weight of the entire lens barrel can only be reduced to a limited extent. In contrast, a zoom lens which has a so-called shift type of image-shake correction means for correcting an image shake by moving a predetermined lens group in a plane perpendicular to the optical axis of the zoom lens is disclosed in Japanese Patent No. 2560377 and others. In the zoom lens having this shift type of image-shake correction means, the lens group required to focus an image can also be used as a shift lens for the image-shake correction means. Therefore, such zoom lens is advantageous in terms of further reductions in the entire length, size and weight, as compared with at least the system using the aforesaid variable angle prism.
However, in an actual lens barrel, actuators such as an IG meter for driving an iris, a zoom motor and a focusing motor are indispensable, and it is, therefore, necessary to reduce the sizes and the weights of these actuators if the entire lens barrel is to be reduced in size and weight.